1. Field of the Invention
This invention relates to an improvement of a magnetic recording medium provided with at least one layer of obliquely deposited magnetic crystals wherein ferromagnetic columnar grains are formed on a substrate at an oblique angle with respect to the substrate by causing a stream of vapor obtained by heating and evaporating a ferromagnetic material to obliquely impinge upon the substrate in a vacuum.
2. Description of the Prior Art
Recently, there is desired a magnetic recording medium able to meet the strong demand for high density recording. Particularly, various efforts are being made to develop a thin metal film type magnetic recording medium suitable as a high density magnetic recording medium. To form a thin metal film, various methods such as vacuum deposition, ion plating, sputtering and wet plating are used. However, as a method of consistently making a high coercive force medium suitable for high density recording, an oblique incidence vacuum deposition method as disclosed in Japanese Patent Publication No. 41(1966)-19389 is excellent.
In the oblique incidence vacuum deposition method, a stream of vapor obtained by heating and evaporating a ferromagnetic material such as Co, or Co-Ni is caused to impinge upon a non-magnetic substrate made of a high-molecular material or the like in a vacuum generally at an angle of 45.degree. or more with respect to the line normal to the substrate, thereby forming ferromagnetic columnar grains on the substrate at an oblique angle with respect to the substrate. In this manner, at least one layer of obliquely deposited magnetic crystals is formed on the substrate.
FIG. 1 shows a continuous deposition apparatus used to make a magnetic tape by use of the aforesaid oblique incidence vacuum deposition method. In this apparatus, the minimum angle of incidence (.theta. min) which is determined by the shapes and positions of a cylindrical cooling can 1, a deposition material 2 and a mask 3 is a very important factor determining the magnetic characteristics, particularly the coercive force Hc, of the obliquely deposited magnetic layer. For example, in order to obtain a high coercive force of about 1,000 Oe by use of Co or a Co-Ni alloy as the deposition material, it is generally necessary that the minimum angle of incidence (.theta. min) be about 60.degree. or more. However, when deposition is conducted at a minimum angle of incidence (74 min) of 60.degree. or more, the portion of the vapor stream interrupted by the mask 3 is increased, and the amount of the vapor stream deposited on the substrate is decreased markedly, resulting in a marked decrease in the deposition efficiency and deposition speed.
To eliminate the drawbacks of the aforesaid oblique incidence vacuum deposition method, it has been proposed to conduct oblique incidence vacuum deposition in the presence of oxygen or to add an impurity to the deposition material. In the method of conducting oblique incidence vacuum deposition in the presence of oxygen, it is possible to obtain a deposited magnetic layer exhibiting a high coercive force and, consequently, decrease the minimum angle of incidence (.theta. min) in the deposition process. Namely, this method is suitable for increasing the coercive force of the obtained magnetic layer and decreasing the angle of incidence of the vapor stream of the deposition material. However, the Co or Co-Ni alloy film formed by the oblique incidence vacuum deposition method or the method of oblique incidence vacuum deposition in the presence of oxygen exhibits insufficient time stability, which constitutes an obstacle to putting the method to practical use. In order to increase the coercive force and improve the time stability of the Co or Co-Ni alloy film, an attempt has been made to add an impurity such as Cr to the Co or Co-Ni alloy deposition material. However, it is not yet possible to realize a sufficient improvement in the time stability. For example, when the amount of Cr added as an impurity is increased, the apparent time stability of the Co or Co-Ni alloy film obtained is somewhat improved. However, in order to obtain a desired level of time stability, it is necessary to add large amounts of Cr to the deposition material. As a result, the magnetic flux density and the electromagnetic transducing characteristics of the obtained magnetic layer become very low.